A weather fluke caused a catastrophic die-off.

We’re used to getting sick when the weather turns cold, but for a population of rare antelopes in Kazakhstan, higher temperatures were deadly. Between May and June 2015, a warm weather epidemic wiped out about 200,000 saiga antelopes (Saiga tatarica tatarica). If this sounds like a high death toll to you, then you’re onto something: That number actually represents about 62 percent of the global population of saiga antelope, a critically endangered species.

In a paper published Wednesday in Science Advances, an international team of researchers investigated the various elements behind this mass die-off. The paper’s authors report that the culprit in this mass mortality event was hemorrhagic septicemia caused by the bacterium Pasteurella multocida type B. They blame unseasonably high temperatures and humidity over just a couple of days for spurring an outbreak of the bacterium. Though the weather event was just a fluctuation, the results were devastating.

“I had never seen anything like it,” Richard Kock, a professor at the Royal Veterinary College in the U.K. and first author on the paper, told The Verge. “It was very concerning because it was so unnatural, outside of the realm of my experience.”

Over three weeks in 2015, over 60 percent of the Earth's saiga antelopes died from a bacterial infection.

Kock and his colleagues pieced together the available evidence to figure out, to the best of their knowledge, why this die-off happened. They watched the antelopes as the herds got sick and individuals began to die, which happened over the course of about nine days at two separate sites.

“Most died within a few hours of onset of clinical signs,” the study’s authors write.

They took some blood samples from sick animals, but once all the animals had died, the researchers got down to their grisly task and took tissue samples — brain, gut, blood, and feces. They sent samples to multiple laboratories for analysis, and by combining the physical pathology of the animals with bacterial cultures, they identified their culprit: *P. multocida type B. This information, coupled with statistical models of the central Kazakhstan weather leading to the die-off, led the researchers to conclude that high temperatures and humidity had created optimal conditions for the bacterium to spread, causing the unwitting antelopes to die painful deaths from hemorrhagic septicemia.

Hemorrhagic septicemia, a condition arising from bacterial infections by members of the Pasteurella genus, often causes death within 24 hours. Symptoms include pneumonia and internal bleeding, as well as lethargy, all symptoms observed by the study’s authors. Even among human-raised livestock, the condition is often fatal because of the rapid onset and progression of the disease. This is especially true for free-ranging herds.

Saiga antelopes aren’t livestock, though. And in fact, they’re incredibly susceptible to mass mortality events like the 2015 one. The species’ sensitivity to mass die-offs actually helped the researchers narrow down the causes of the epidemic, as they could compare temperatures in 2015 to those that occurred during a 1988 mass mortality event that also occurred among the saiga antelopes. Back then, the culprit was also suspected to be bacteria-related. Temperature and humidity had been the differentiating factors in 1988, too.

Scientists say this research can help inform future conservation efforts for this sensitive species. Since it’s likely that mass mortality events will happen again, the paper’s authors say future wildlife management efforts must take this into account.

“This requires investment in preventative measures where possible (for example, livestock vaccination for PPR), strong antipoaching actions so that populations are large and resilient enough to withstand large-scale mortality, and a sustainable landscape-level approach to coexistence between saigas, livestock, and people to enable its migratory lifestyle to continue.”

Abstract:

In 2015, more than 200,000 saiga antelopes died in 3 weeks in central Kazakhstan. The proximate cause of death is confirmed as hemorrhagic septicemia caused by the bacterium *Pasteurella multocida type B, based on multiple strands of evidence. Statistical modeling suggests that there was unusually high relative humidity and temperature in the days leading up to the mortality event; temperature and humidity anomalies were also observed in two previous similar events in the same region. The modeled influence of environmental covariates is consistent with known drivers of hemorrhagic septicemia. Given the saiga population’s vulnerability to mass mortality and the likely exacerbation of climate-related and environmental stressors in the future, management of risks to population viability such as poaching and viral livestock disease is urgently needed, as well as robust ongoing veterinary surveillance. A multidisciplinary approach is needed to research mass mortality events under rapid environmental change.